Following on from doing a wither trace. When the seller tries it in the saddle, the trace needs to be set about an inch in from the front so it sits under the fork/bars of the tree. Where you took the measurement of your horse from.
Ideally your looking for a saddle that makes contact both sides of your trace, as much contact as possible but leaving out the top of the wither.
This is a example of a trace working well in a saddle and it fitted the horse when tried on.
Again just like when your trying the saddle on. You want the angle of your horses trace to follow the angle of the saddle.
If your horse is very uneven (99% of horses are uneven a little, just like humans.) but if there noticeably uneven like the below example, it would be impossible for a saddle to follow both sides equally. So instead try to get the angles to match the angle of your horse. You then may need to shim the less muscled side.
Again getting the width right is only 1 factor of fit. The other 2 main factors to consider are:
Tree shape matching your horses back shape. See my other blog post entitled "rock" for further information on this.
These are examples of Incorrect bar angles. If a perfect fit isn't achievable due to budget or the horse is beyond range of "normal" shape. Then go for a fit that won't cause pressure points on your horse.
This is what to look out for:
Traces not working:
Too wide (see how it's only making contact lower down)
Trace saddle too narrow: